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08:36:59">2022-08-24</time><span class="post-meta-separator">|</span><i class="fas fa-history fa-fw post-meta-icon"></i><span class="post-meta-label">更新于</span><time class="post-meta-date-updated" datetime="2022-08-24T00:37:22.791Z" title="更新于 2022-08-24 08:37:22">2022-08-24</time></span></div><div class="meta-secondline"><span class="post-meta-separator">|</span><span class="post-meta-wordcount"><i class="far fa-file-word fa-fw post-meta-icon"></i><span class="post-meta-label">字数总计:</span><span class="word-count">524</span><span class="post-meta-separator">|</span><i class="far fa-clock fa-fw post-meta-icon"></i><span class="post-meta-label">阅读时长:</span><span>1分钟</span></span><span class="post-meta-separator">|</span><span class="post-meta-pv-cv" id="" data-flag-title="Promis_Async_await_Loop"><i class="far fa-eye fa-fw post-meta-icon"></i><span class="post-meta-label">阅读量:</span><span id="busuanzi_value_page_pv"></span></span></div></div></div><article class="post-content" id="article-container"><ol>
<li>抽象表达<ol>
<li>Promise是一个新的技术(Es6规范)</li>
<li>Promise是Js中进行异步编程的新解决方案(旧方案是单纯使用回调函数)</li>
</ol>
</li>
<li>具体表达:<ol>
<li>语法层面:Promise是一个构造函数</li>
<li>功能方面:Promise对象用来封装一个异步操作并可以获取其 成功/失败 的结果值</li>
</ol>
</li>
</ol>
<h4 id="异步编程有那些"><a href="#异步编程有那些" class="headerlink" title="异步编程有那些"></a>异步编程有那些</h4><ul>
<li><p>fs 文件操作 nodejs</p>
<figure class="highlight plaintext"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line">require(&#x27;fs&#x27;).readFile(&#x27;XXX&#x27;,(err,data)=&gt;&#123;&#125;)</span><br></pre></td></tr></table></figure></li>
<li><p>数据库操作</p>
</li>
<li><p>Ajax</p>
<figure class="highlight plaintext"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line">$.get(&#x27;/server&#x27;,(data)=&gt;&#123;&#125;)</span><br></pre></td></tr></table></figure></li>
<li><p>定时器</p>
<figure class="highlight plaintext"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line">setTimeout(()=&gt;&#123;&#125;,2000)</span><br></pre></td></tr></table></figure></li>
</ul>
<h3 id="为什么使用Promise"><a href="#为什么使用Promise" class="headerlink" title="为什么使用Promise"></a>为什么使用Promise</h3><ol>
<li><p>指定回调函数的方式灵活</p>
<ol>
<li>旧的:必须启动异步任务前指定</li>
<li>promise:启动异步任务=&gt;返回promise对象=&gt;给promise对象绑定回调函数(甚至可以异步任务结束后指定多个回调)</li>
</ol>
</li>
<li><p>支持<strong>链式调用</strong>,可以解决<strong>回调地狱</strong>的问题</p>
<ol>
<li><p>回调地狱是什么?</p>
<p>回调函数里边还有回调函数(不断缩进)</p>
</li>
<li><p>回调地狱的特点</p>
<ul>
<li>不便于阅读</li>
<li>不便于异常处理</li>
</ul>
</li>
<li><p>解决方案</p>
<ul>
<li>promise链式调用</li>
</ul>
</li>
</ol>
</li>
</ol>
<p>reject 是 Promise 的方法,而 catch 是 Promise 实例的方法”<br>reject 是用来抛出异常，catch 是用来处理异常<br>reject后的东西，一定会进入then中的第二个回调，如果then中没有写第二个回调，则进入catch<br><img src= "https://cdn.jsdelivr.net/gh/Cai2w/cdn/img/loading.gif" data-lazy-src="C:\Users\20815\AppData\Roaming\Typora\typora-user-images\image-20220815175824784.png" alt="image-20220815175824784"></p>
<h2 id="事件循环"><a href="#事件循环" class="headerlink" title="事件循环"></a>事件循环</h2><p>promise算作同步任务,其中实例调用then,对象调用的rej()算异步任务</p>
<p><img src= "https://cdn.jsdelivr.net/gh/Cai2w/cdn/img/loading.gif" data-lazy-src="C:\Users\20815\AppData\Roaming\Typora\typora-user-images\image-20220815193930507.png" alt="image-20220815193930507"></p>
<p>宏任务里面如果有微任务的话一定要等待这个微任务执行完毕之后才会执行下个宏任务，然后同级队列情况是先访问微任务队列在访问宏任务队列，</p>
<p><img src= "https://cdn.jsdelivr.net/gh/Cai2w/cdn/img/loading.gif" data-lazy-src="C:\Users\20815\AppData\Roaming\Typora\typora-user-images\image-20220815201916096.png" alt="image-20220815201916096"></p>
<h2 id="Async-await"><a href="#Async-await" class="headerlink" title="Async await"></a>Async await</h2><p>调用Async函数时,内部同步顺序执行,await后边的所有东西等待返回后顺序执行, await后边的全部踢入微任务</p>
<figure class="highlight js"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">async</span> <span class="function"><span class="keyword">function</span> <span class="title">a</span>(<span class="params"></span>) </span>&#123;</span><br><span class="line">            <span class="built_in">console</span>.log(<span class="number">1</span>)</span><br><span class="line">            <span class="built_in">setTimeout</span>(<span class="function">() =&gt;</span> &#123;</span><br><span class="line">                <span class="built_in">console</span>.log(<span class="number">4</span>)</span><br><span class="line">            &#125;,)</span><br><span class="line">            <span class="keyword">let</span> c = <span class="keyword">await</span> <span class="number">3</span></span><br><span class="line">            <span class="built_in">console</span>.log(c)</span><br><span class="line">            <span class="built_in">console</span>.log(<span class="number">5</span>)</span><br><span class="line">            <span class="built_in">setTimeout</span>(<span class="function">() =&gt;</span> &#123;</span><br><span class="line">                <span class="built_in">console</span>.log(<span class="number">6</span>)</span><br><span class="line">            &#125;)</span><br><span class="line">&#125;</span><br><span class="line">        <span class="built_in">console</span>.log(<span class="number">8</span>)</span><br><span class="line">        a()</span><br><span class="line">        <span class="built_in">setTimeout</span>(<span class="function">() =&gt;</span> &#123;</span><br><span class="line">            <span class="built_in">console</span>.log(<span class="number">7</span>)</span><br><span class="line">        &#125;)</span><br><span class="line">        <span class="built_in">console</span>.log(<span class="number">2</span>)</span><br></pre></td></tr></table></figure>

<p>81235476</p>
</article><div class="post-copyright"><div class="post-copyright__author"><span class="post-copyright-meta">文章作者: </span><span class="post-copyright-info"><a href="mailto:undefined">lee</a></span></div><div class="post-copyright__type"><span class="post-copyright-meta">文章链接: </span><span class="post-copyright-info"><a href="http://blog.cai2.wang/2022/08/24/Promis-Async-await-Loop/">http://blog.cai2.wang/2022/08/24/Promis-Async-await-Loop/</a></span></div><div class="post-copyright__notice"><span class="post-copyright-meta">版权声明: </span><span class="post-copyright-info">本博客所有文章除特别声明外，均采用 <a href="https://creativecommons.org/licenses/by-nc-sa/4.0/" target="_blank">CC BY-NC-SA 4.0</a> 许可协议。转载请注明来自 <a href="http://blog.cai2.wang" target="_blank">李世龙</a>！</span></div></div><div class="tag_share"><div class="post-meta__tag-list"></div><div class="post_share"><div class="social-share" data-image="https://cdn.jsdelivr.net/gh/Cai2w/cdn/img/undraw_pair_programming_njlp.png" data-sites="facebook,twitter,wechat,weibo,qq"></div><link rel="stylesheet" href="https://cdn.jsdelivr.net/npm/social-share.js/dist/css/share.min.css" media="print" onload="this.media='all'"><script src="https://cdn.jsdelivr.net/npm/social-share.js/dist/js/social-share.min.js" defer></script></div></div><div class="post-reward"><div class="reward-button button--animated"><i class="fas fa-qrcode"></i> 打赏</div><div class="reward-main"><ul class="reward-all"><li class="reward-item"><a href="https://cdn.jsdelivr.net/gh/Cai2w/cdn/img/wechat.jpg" target="_blank"><img class="post-qr-code-img" src="https://cdn.jsdelivr.net/gh/Cai2w/cdn/img/wechat.jpg" alt="微信"/></a><div class="post-qr-code-desc">微信</div></li><li class="reward-item"><a href="https://cdn.jsdelivr.net/gh/Cai2w/cdn/img/alipay.jpg" target="_blank"><img class="post-qr-code-img" src="https://cdn.jsdelivr.net/gh/Cai2w/cdn/img/alipay.jpg" alt="支付宝"/></a><div class="post-qr-code-desc">支付宝</div></li></ul></div></div><nav class="pagination-post" id="pagination"><div class="prev-post pull-left"><a href="/2022/08/24/Vue%E5%85%A8%E5%AE%B6%E6%A1%B6/"><img class="prev-cover" src="https://cdn.jsdelivr.net/gh/Cai2w/cdn/img/undraw_Mind_map_re_nlb6.png" onerror="onerror=null;src='https://cdn.jsdelivr.net/gh/Cai2w/cdn/img/404.jpg'" alt="cover of previous post"><div class="pagination-info"><div class="label">上一篇</div><div class="prev_info">Vue全家桶</div></div></a></div><div class="next-post pull-right"><a href="/2022/08/23/Js%E5%B8%B8%E7%94%A8%E6%95%B0%E7%BB%84/"><img class="next-cover" src="https://cdn.jsdelivr.net/gh/Cai2w/cdn/img/undraw_dev_productivity_umsq.png" onerror="onerror=null;src='https://cdn.jsdelivr.net/gh/Cai2w/cdn/img/404.jpg'" alt="cover of next post"><div class="pagination-info"><div class="label">下一篇</div><div class="next_info">Js常用数组</div></div></a></div></nav></div><div class="aside-content" id="aside-content"><div class="sticky_layout"><div class="card-widget" id="card-toc"><div class="item-headline"><i class="fas fa-stream"></i><span>目录</span></div><div class="toc-content"><ol class="toc"><li class="toc-item toc-level-4"><a class="toc-link" href="#%E5%BC%82%E6%AD%A5%E7%BC%96%E7%A8%8B%E6%9C%89%E9%82%A3%E4%BA%9B"><span class="toc-number">1.</span> <span class="toc-text">异步编程有那些</span></a></li></ol></li><li class="toc-item toc-level-3"><a class="toc-link" href="#%E4%B8%BA%E4%BB%80%E4%B9%88%E4%BD%BF%E7%94%A8Promise"><span class="toc-number"></span> <span class="toc-text">为什么使用Promise</span></a></li></ol></li><li class="toc-item toc-level-2"><a class="toc-link" href="#%E4%BA%8B%E4%BB%B6%E5%BE%AA%E7%8E%AF"><span class="toc-number"></span> <span class="toc-text">事件循环</span></a></li><li class="toc-item toc-level-2"><a class="toc-link" href="#Async-await"><span class="toc-number"></span> <span class="toc-text">Async await</span></a></div></div></div></div></main><footer id="footer"><div id="footer-wrap"><div class="copyright">&copy;2021 - 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